The influence of intracrystalline diffusion on the selective catalytic reduction of NO was examined by using cobalt-exchanged mordenites with different crystal sines. In the NO-CH4-O-2 reaction, the reaction rate increased with cobalt exchange level, but the rate over a large crystal mordenite was slightly lower than that over a small crystal mordenite. In the NO-C3H8-O-2 reaction, where the reaction rate was higher than that of the NO-CH4-O-2 reaction, the effect of crystal size was more significant. Furthermore, the reaction rate over the small crystal increased with increasing cobalt exchange level, whereas the rate over the large crystal decreased with increasing cobalt exchange level. The effectiveness factor calculated from these results was below unity, indicating that the volume of zeolite crystal was not fully utilized because of the diffusion limitations.